Wireless LAN with load balancing

ABSTRACT

A communication system with a plurality of access points (AP 1 , AP 2 , AP 3 ) and at least one network station ( 5, 6 ), the network station ( 5, 6 ) being arranged to communicate with one of said plurality of access points (AP 1 , AP 2 , AP 3 ) through a wireless communication protocol, each access point (AP 1 , AP 2 , AP 3 ) is able to:  
     monitor its access point traffic load and  
     transmit an access point traffic load parameter (ATT) to the network station ( 5, 6 ), and the network station ( 5, 6 ) is able to:  
     monitor its network station traffic load;  
     store a network station traffic load parameter (AUTT);  
     receive access point traffic load parameters (ATT) from the access points (AP 1 , AP 2 , AP 3 );  
     select a communication connection with one of the access points (AP 1 , AP 2 , AP 3 ) using a predetermined cost function taking the access point traffic load parameters (ATT) and the network station traffic load parameters (AUTT) into account.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority of European Patent ApplicationNo. 00304239.7, which was filed on May 19, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to communication system comprisinga plurality of access points and at least one network station, said atleast one network station being arranged to communicate with one of saidplurality of access points through a wireless communication protocol.

PRIOR ART

[0003] Recently, wireless local area networks (LANs) have been developedas an enhanced replacement for wired LANs. In a wireless LAN fordata-communication a plurality of (mobile) network stations (e.g.,personal computers, telecommunication devices, etc.) are present thatare capable of wireless communication. As compared to wired LANs,data-communication in a wireless LAN can be more versatile, due to theflexibility of the arrangement of network stations in the area coveredby the LAN, and due to the absence of cabling connections.

[0004] Wireless LANs are generally implemented according to the standardas defined by the ISO/IEC 8802-11 international standard (IEEE 802.11).IEEE 802.11 describes a standard for wireless LAN systems that willoperate in the 2.4-2.5 GHz ISM (industrial, scientific and medical)band. This ISM band is available worldwide and allows unlicensedoperation for spread spectrum systems. For both the US and Europe, the2,400-2,483.5 MHz band has been allocated, while for some othercountries, such as Japan, another part of the 2.4-2.5 GHz ISM band hasbeen assigned. The IEEE 802.11 standard focuses on the MAC (mediumaccess control) and PHY (physical layer) protocols for access pointbased networks and ad-hoc networks.

[0005] In access point based wireless networks, the stations within agroup or cell can communicate only directly to the access point. Thisaccess point forwards messages to the destination station within thesame cell or through the wired distribution system to another accesspoint, from which such messages arrive finally at the destinationstation. In ad-hoc networks, the stations operate on a peer-to-peerlevel and there is no access point or (wired) distribution system.

[0006] The 802.11 standard supports three PHY protocols: DSSS (directsequence spread spectrum), FHSS (frequency hopping spread spectrum), andinfrared with PPM (pulse position modulation). All these three PHYs allprovide bit rates of 1 and 2 Mbit/s. Furthermore, IEEE 802.11 includesextensions 11 a and 11 b which allow for additional higher bit rates:Extension 11 b provides bit rates 5.5 and 11 Mbit/s as well as the basicDSSS bit rates of 1 and 2 Mbit/s within the same 2.4-2.5 GHz ISM band.Extension 11 a provides a high bit rate OFDM (Orthogonal FrequencyDivision Multiplexing modulation) PHY standard providing bit rates inthe range of 6 to 54 Mbit/s in the 5 GHz band.

[0007] The IEEE 802.11 basic MAC protocol allows interoperabilitybetween compatible PHYs through the use of the CSMA/CA (carrier sensemultiple access with collision avoidance) protocol and a random back-offtime following a busy medium condition. The IEEE 802.11 CSMA/CA protocolis designed to reduce the collision probability between multiplestations accessing the medium at the same time. Therefore, a randomback-off arrangement is used to resolve medium contention conflicts. Inaddition, the IEEE 802. 11 MAC protocol defines special functionalbehaviour for fragmentation of packets, medium reservation via RTS/CTS(request-to-send/clear-to-send) polling interaction and pointcoordination (for time-bounded services).

[0008] Moreover, the IEEE 802.11 MAC protocol defines Beacon frames sentat regular intervals by the access point to allow stations to monitorthe presence of the access point. The IEEE 802.11 MAC protocol alsogives a set of management frames including Probe Request frames whichare sent by a station and are followed by Probe Response frames sent byan available access point, to allow a station to scan actively if thereis an access point operating on a certain channel frequency and to showto the station what parameter settings this access point is using.

[0009] In a wireless local area network, most stations are mobile: theymay move around, more or less freely, in the network area. The rangecovered by a specific access point is however limited: the cell sizearound an access point is defined for the (expected) requirements of the(average) number of stations associated with the cell and the amount ofnetwork traffic these stations generate: Cell sizes vary from smallwhere a high traffic density is anticipated, to large for a low trafficdensity. Also, the cell size is limited by physical constraints.

[0010] Thus, in order to stay connected to the network, mobile stationsmay need to change their association from one access point to anotherwhen the reception level of the associated access point becomes too low.The choice by the station to switch between access points is based onthe station's relative reception levels of the access points involved.This procedure is known in the art as roaming.

[0011] In the prior art, this roaming procedure is based on the selectedconfiguration setting for one of the different access point (AP) densitysettings which influence defer, carrier detect and cell searchbehaviour. A network station can be configured for cell sizes in a scalefrom large to small with corresponding AP density levels ranging fromlow to high. When the network station is configured for a large cellsize (AP density low), it will allow more degradation in the receptionconditions of the connection to the access point before it startslooking for an access point which can be received better. When in thiscase, for example, the access point is configured for a small cell size(AP density high), the network station will start looking for an accesspoint which can be received better.

[0012] European patent application . filed on . (Lucent's reference: A.Eikelenboom 1-11-10/IDS no. 120712), which is prior art under Art. 53(3)/(4) EPC, describes the method that network stations use to(re)configure their settings relating to cell size and AP densityautomatically, depending on the access point to which they associate.

[0013] However, the traffic load of the cells is not taken into accountin this roaming procedure. It is possible that the station disassociatesfrom an access point with a low traffic load, and associates withanother access point that has a relatively high traffic load. By joiningthe cell of this access point, its traffic load may even increasefurther, which may result in a degradation of the traffic within thelatter cell (by traffic between stations within the cell) and even inthe complete network (by traffic between stations in the cell andstations outside of it). Also, it is possible that a network station isassociated with an access point with a high traffic load and a goodsignal reception level, while other access points may have lower trafficload and acceptable signal reception levels. When traffic load is nottaken into account, the network station will not switch over to anotheraccess point and the load in the network can become unbalanced.

SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to improve the roamingprocedure in such a way that switching (association) of stations fromone cell to another is not only based on signal reception levels butalso on the traffic load of the cells. Furthermore, it is an object ofthe present invention to balance the traffic load of a wireless LAN byredistributing load (i.e., stations) over the cells in the network. Bythe present invention a better overall throughput behaviour for thewireless LAN will be provided.

[0015] The present invention relates to a communication systemcomprising a plurality of access points and at least one networkstation, said at least one network station being arranged to communicatewith one of said plurality of access points through a wirelesscommunication protocol, wherein each of said plurality of access pointsis arranged to:

[0016] monitor its access point traffic load and

[0017] transmit an access point traffic load parameter indicative ofsaid access point traffic load to said at least one network station, andsaid at least one network station is arranged to:

[0018] monitor its network station traffic load;

[0019] store a network station traffic load parameter indicative of saidnetwork station traffic load;

[0020] receive access point traffic load parameters from said pluralityof access points;

[0021] select a communication connection with one of said plurality ofaccess points using a predetermined cost function taking at least saidaccess point traffic load parameters and said network station trafficload parameters into account.

[0022] Moreover, the present invention relates to an access pointarranged to communicate with at least one network station through awireless communication protocol, wherein said access point is alsoarranged to:

[0023] monitor its access point traffic load and

[0024] transmit an access point traffic load parameter indicative ofsaid access point traffic load to said at least one network station.

[0025] Furthermore, the present invention relates to a network stationarranged to communicate with one of a plurality of access points througha wireless communication protocol, wherein said network station isarranged to

[0026] monitor its network station traffic load;

[0027] store a network station traffic load parameter indicative of saidnetwork station traffic load;

[0028] receive access point traffic load parameters indicative of accesspoint traffic loads from said plurality of access points;

[0029] select a communication connection with one of said plurality ofaccess points using a predetermined cost function taking at least saidaccess point traffic load parameters and said network station trafficload parameters into account.

[0030] Also, the present invention relates to a method in acommunication system comprising a plurality of access points and atleast one network station for communication between said at least onenetwork station and one of said plurality of access points, said atleast one network station being arranged to communicate with one of saidplurality of access point through a wireless communication protocol,wherein said method comprises the following steps carried out by theaccess points:

[0031] monitoring access point traffic loads and

[0032] transmitting access point traffic load parameters indicative ofsaid access point traffic loads to said at least one network station,

[0033] and said method also includes the following steps carried out bysaid at least one network station:

[0034] monitoring network station traffic load;

[0035] storing a network station traffic load parameter indicative ofsaid network station traffic load;

[0036] receiving access point traffic load parameters from saidplurality of access points;

[0037] selecting a communication connection with one of said pluralityof access points using a predetermined cost function taking at leastsaid access point traffic load parameters and said network stationtraffic load parameters into account.

[0038] Moreover, the present invention relates to a method carried outby an access point arranged to communicate with at least one networkstation through a wireless communication protocol, wherein said methodcomprises the following steps:

[0039] monitoring access point traffic load and

[0040] transmitting an access point traffic load parameter indicative ofsaid access point traffic load to said at least one network station.

[0041] Furthermore, the present invention relates to a computer programproduct to be loaded by an access point arranged to communicate with atleast one network station (5, 6) through a wireless communicationprotocol, wherein said computer program product allows said access pointto carry out the following steps:

[0042] monitoring access point traffic load and

[0043] transmitting an access point traffic load parameter indicative ofsaid access point traffic load to said at least one network station.

[0044] The present invention relates also to a data carrier providedwith a computer program product to be loaded by an access point asdefined above

[0045] Moreover, the present invention relates to a method to be carriedout by a network station arranged to communicate with one of a pluralityof access points through a wireless communication protocol, wherein saidmethod comprises the following steps:

[0046] monitoring network station traffic load;

[0047] storing a network station traffic load parameter indicative ofsaid network station traffic load;

[0048] receiving access point traffic load parameters indicative ofaccess point traffic loads from said plurality of access points;

[0049] selecting a communication connection with one of said pluralityof access points using a predetermined cost function taking at leastsaid access point traffic load parameters and said network stationtraffic load parameters into account.

[0050] Furthermore, the present invention relates to a computer programproduct to be loaded by a network station arranged to communicate withone of a plurality of access points through a wireless communicationprotocol, wherein said computer program product allows said networkstation to carry out the following steps:

[0051] monitoring network station traffic load;

[0052] storing a network station traffic load parameter indicative ofsaid network station traffic load;

[0053] receiving access point traffic load parameters indicative ofaccess point traffic loads from said plurality of access points;

[0054] selecting a communication connection with one of said pluralityof access points using a predetermined cost function taking at leastsaid access point traffic load parameters and said network stationtraffic load parameters into account.

[0055] Also, the present invention relates to a data carrier providedwith a computer program product to be loaded by a network station asdefined above.

[0056] By the present invention a better overall throughput behaviourfor the wireless LAN will be provided.

BRIEF DESCRIPTION OF THE DIAGRAMS

[0057] Below, the invention will be explained with reference to somedrawings, which are intended for illustration purposes only and not tolimit the scope of protection as defined in the accompanying claims.

[0058]FIG. 1 shows a schematic overview of a wireless LAN comprising aplurality of network cells and a network station to illustrate theinvention;

[0059]FIG. 2a shows a schematic block diagram of a network station inthe present invention;

[0060]FIG. 2b shows a schematic block diagram of an access point of thepresent invention;

[0061]FIG. 2c shows a schematic example of a wireless LAN interface formedium access controller devices as described in this invention for usein a network station or an access point;

[0062]FIG. 3 shows diagrammatically the extended load balancinginformation data frame used in the Beacon and Probe Response framesaccording to the present invention;

[0063]FIG. 4 shows a flow diagram to illustrate the method of roaming inthe present invention;

[0064]FIG. 5 shows a flow diagram to illustrate the method of loadbalancing in the present invention;

[0065]FIGS. 6a-6 f are a schematic illustration of the effect on theroaming behaviour of network stations in a network configuration, withand without load balancing, according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0066]FIG. 1 shows a schematic overview of a wireless LAN 1 in which theinvention is implemented. The wireless LAN 1 includes a number of accesspoints of which three access points are shown AP1, AP2, AP3. Theseaccess points serve as access point for their respective cells 2, 3, 4which are each schematically depicted by a circle around theirrespective access point. In the LAN 1 a plurality of network stations 5,6 is present of which two are shown. Each access point AP1, AP2, AP3 isconnected (not shown) via suitable I/O means to a wired distributionnetwork for communication with other access points. In LAN 1 the actualnumber of network stations 5 may be 0 (zero) or more. The networkstations 5, 6 may be mobile or at fixed positions: they all connect tothe network 1 by means of wireless data-communication. In thisembodiment of the invention the network stations 5, 6 are represented bypersonal computers, but it is to be understood that the network stationsmay be any type of telecommunication equipment that uses a wirelessdata-communication network, such as mobile telephones, pagers, PDAs(personal digital assistants), laptop computers, etc.

[0067] The cells 2, 3, 4 have different sizes as depicted by the size ofthe circles. The cell size is determined by the requirements of datathroughput in the cell and can be controlled by suitable setting of thelevels of the defer behaviour threshold and carrier sense detectionthreshold (for a cell including all its associated stations or, forstations individually) as known from EP-A-0903891. For example, a cell 2may comprise a number of network stations that require high throughputs.In that case, the cell size should be small such that other networkstations will be left out of the cell as much as possible. In anothercase, for example, in a cell 4 only few network station with lowthroughput requirements will be present. Then, a single large cell 4comprising these network stations will be sufficient to handle all datatraffic related to that cell 4.

[0068]FIG. 2a shows a schematic block diagram of an embodiment of anetwork station 5, 6 comprising processor means 21 with peripherals. Theprocessor means 21 is connected to memory units 18, 22, 23, 24 whichstore instructions and data, one or more reading units 25 (to read,e.g., floppy disks 19, CD ROM's 20, DVD's, etc.), a keyboard 26 and amouse 27 as input devices, and as output devices, a monitor 28 and aprinter 29. Other input devices, like a trackball and a touch screen,and output devices may be provided for. For data-communication over thewireless LAN 1, an interface card 30 is provided. The interface card 30connects to an antenna 31.

[0069] The memory units shown comprise RAM 22, (E)EPROM 23, ROM 24 andhard disk 18. However, it should be understood that there may beprovided more and/or other memory units known to persons skilled in theart. Moreover, one or more of them may be physically located remote fromthe processor means 21, if required. The processor means 21 are shown asone box, however, they may comprise several processing units functioningin parallel or controlled by one main processor, that may be locatedremote from one another, as is known to persons skilled in the art.

[0070] In an alternative embodiment of the present invention, thenetwork station 5, 6 may be a telecommunication device in which thecomponents of interface card 30 are incorporated as known to thoseskilled in the art.

[0071]FIG. 2b shows a schematic block diagram of an embodiment of anaccess point AP1, AP2, AP3 comprising processor means 121 withperipherals. The processor means 121 is connected to memory units 118,122, 123, 124 which store instructions and data, one or more readingunits 125 (to read, e.g., floppy disks 119, CD ROM's 120, DVD's, etc.),a keyboard 126 and a mouse 127 as input devices, and as output devices,a monitor 128 and a printer 129. For data-communication over thewireless LAN 1, an interface card 130 is provided. The interface card130 connects to an antenna 131. Furthermore, the access point AP1, AP2,AP3 is connected to a wired distribution network 140 through I/O means132 for communication with other access points.

[0072] The memory units shown comprise RAM 122, (E)EPROM 123, ROM 124and hard disk 118. However, it should be understood that there may beprovided more and/or other memory units known to persons skilled in theart. Moreover, one or more of them may be physically located remote fromthe processor means 121, if required. The processor means 121 are shownas one box, however, they may comprise several processing unitsfunctioning in parallel or controlled by one main processor, that may belocated remote from one another, as is known to persons skilled in theart. Moreover, other input/output devices than those shown (i.e., 126,127, 128, 129) may be provided.

[0073] In an alternative embodiment of the present invention, the accesspoint AP1, AP2, AP3 may be a telecommunication device in which thecomponents of interface card 130 are incorporated as known to thoseskilled in the art.

[0074]FIG. 2c shows a block diagram of the arrangement of the presentinvention for a medium access controller (MAC) device 301 on a wirelessLAN interface card 30 installed in a network station 5, 6 or on asimilar wireless LAN interface card 130 installed in an access pointAP1, AP2, AP3, respectively.

[0075] Here, the MAC device 301 is schematically depicted, showing onlya signal processing unit 302, a signal reception level detection circuit303, antenna 311 and a on-board memory 304 as needed for the descriptionof this embodiment of the invention. The MAC device 301 may compriseother components not shown here. Also, the components 302, 303, 304which are shown, may be separate devices or integrated into one device.As desired, the devices also may be implemented in the form of analog ordigital circuits. The on-board memory 304 may comprise RAM, ROM,FlashROM and/or other types of memory devices, as are known in the art.

[0076] During operation, a network station is associated with one of theaccess points in the network. As defined by IEEE 802.11, when a networkstation enters a network cell, it engages in a connection with thecell's access point through an initialisation process known asassociation. In FIG. 1, for example, the network station 5 is connectedto the access point AP1 for all its data traffic. The network station 5continuously monitors the communication quality (i.e. the differencebetween the signal reception level and the average noise level) of itsconnection to the access point AP1. As long as a good communicationquality for the associated access point AP1 is maintained, the networkstation 5 stays connected to AP1. When the communication qualitydecreases below a predetermined level (i.e. determined by the AP densitylevel settings the station 5 received during the association with AP1),the network station 5 starts to search for a cell 3, 4 (an access pointAP2, AP3) with a better communication quality. To this purpose, thenetwork station 5 is probing the associated access point AP1 and allother access points AP2, AP3 within range, as known to persons skilledin the art. In this procedure the network station 5 uses the signalreception level of the Beacon frames received from the associated accesspoint AP1 and the Probe Response frames from those other access pointsAP2, AP3. The Probe Response frames are received by the network station5 following Probe Request frames sent by the network station 5. As knownfrom IEEE 802.11, the other access points AP2, AP3 may be operating onchannels with other frequencies than the associated access point AP1.

[0077] As in prior art, the network station 5 can derive the averagenoise level (NLST) from the reception of signals through signalreception level detection circuit 303. In the present invention thenetwork station 5 also receives data on the average noise level of anaccess point from an extension of that access point's Beacon and/orProbe response frames which indicate the average noise level (NLAP)observed by the access point.

[0078] Since the average noise level in a transmission may have aspatially asymmetric distribution due to interference, the presentinvention provides a way to account for differences in average noiselevel observed by the network station 5 and by the access point (AP1,AP2, AP3). The network station 5 will choose the largest value of theaverage noise level (i.e. NLAP as observed by the access point or NLSTas observed by the network station) to determine the minimumcommunication quality for that access point.

[0079] Furthermore, in the embodiment of the present invention thedecision of a network station 5 to switch from an access point AP1 toanother access point AP2, AP3 is dependent not only on the communicationquality of each respective access point but also on the traffic load ofeach access point and of the network station itself. The presentinvention provides an extension in the scanning and hand-over process ofa network station 5, 6 based on a combination of the communicationquality (signal reception level and average noise level) and the load(of both the access points AP1, AP2, AP3 available to the station andthe station 5, 6 itself). The access points AP1, AP2, AP3 monitor theirtraffic load (modem utilisation), preferably, by keeping record of theAverage TX/RX Activity Time (ATT) value averaged over a certain timeinterval (e.g. 10s) (TX/RX: transmission/reception). In the MAC device301 the signal processing unit 302 monitors the amount of all datatransmitted and received by the access point. The ATT value isaccumulated in the on-board memory device 304. The network station 5, 6keeps record of the Average Unicast TX/RX Activity Time (AUTT) valueaveraged over a certain time interval (e.g. 10s). This value representsthe traffic load of the network station (5, 6) itself. In the MAC device301 the signal processing unit 302 monitors the amount of all datatransmitted and received by the network station. The AUTT value isaccumulated in the on-board memory device 304.

[0080] Both the ATT and AUTT values could be represented by a number inthe range from 0 to 100, and could be regarded as a percentage of themodem utilisation of access point and network station, respectively. Itwill be understood that other representations of these values may beused as well.

[0081] Access points in a wireless LAN of the present invention willincorporate the load information as given by the ATT value in the Beaconand Probe Response frames within an additional new data frame. TheBeacon and Probe Response frames are specified according to IEEE 802.11.In the present invention, a LBinfo (Load Balancing Information) dataframe extends both frames and follows directly after the standard IEEE802.11 Beacon or Probe Response frame.

[0082] In FIG. 3 an exemplary block diagram of the layout of the LBinfodata frame is given. The LBinfo data frame comprises in this order:Element ID (1 byte), Length (1 byte), OUI (3 bytes), Load (1 byte) andAverage noise level (1 byte).

[0083] The Element ID identifies the data frame, which, in this case, isthe LBInfo data frame. The Length byte yields the number of bytes in theremainder of the data frame. The OUI bytes contain an OrganisationallyUnique Identifier for the data frame. In the Load element of LBinfo, theaccess point transmits the actual ATT value. In the element Averagenoise level (NLAP) the access point transmits an indication of theactual noise level, experienced by the access point.

[0084] It should be noted that, for reasons of compatibility, the IEEE802.11 specifies that additional new data frames following directlyafter the standard IEEE 802.11 Beacon frame or Probe Response frameshould be ignored by hardware (i.e., access points and network stations)that lacks the appropriate implementation for the new data frames Thus,in this embodiment, the present invention is compatible with existingdevices from the prior art.

[0085] Furthermore, it should be noted that in FIG. 3 the LBInfo dataframe consists of just seven bytes. It may however be possible that theLBInfo data frame comprises more bytes to transmit additionalinformation regarding the load of the access point.

[0086] During roaming, in an operation mode known as the connected scanstate, a network station 5 that is associated with an access point AP1,will update the information on the connection with AP1 on a regularbasis (e.g. every 2s). From the LBinfo data frame in the Beacon framefrom the access point AP1, the network station 5 can ascertain anindication of the communication quality and the load of the access pointAP1: the Load of AP1 (ATT) is transmitted by the access point AP1.

[0087] The network station 5 has accumulated information on its datatraffic load (AUTT). Through its signal reception level detectioncircuit 303, the network station 5 has information on the signalreception level (SRL) and the average noise level (NLST) as observed bythe network station 5.

[0088] The network station 5 chooses the largest value (ANL) for theaverage noise level (i.e. average noise level NLAP of the access pointor average noise level NLST of the network station) for furthercalculations.

[0089] The network station can now evaluate the connection bycalculation of a combined communication quality and load value (CQL)which is formed through a cost function using the values of SRL, ANL,and ATT. The cost function may be a linear function, but othernon-linear functions are possible as well. Also, the cost function mayuse a weight factor WEIGHT to modify the relative influence ofcommunication quality and load in the cost function. The value of WEIGHTmay be a function of the connection or scan state of the network station5 (i.e. connected scan state, searching scan state: for a new accesspoint, or out-of-range scan state: not connected to any access point).For example, the relative weight of the traffic load in the calculationof CQL may be lower in the searching scan state than in the connectedscan state, and may be the lowest while in the out-of-range scan state.Also the network station 5 may derive a threshold value ThV from the APdensity setting (which is also transmitted by the access point in theBeacon frame).

[0090] On the basis of the CQL value calculated in the cost function andthe threshold value ThV, the network station 5 can (decide to) changeits association with access point AP1 and search for other availableaccess points AP2, AP3.

[0091] Similarly to this procedure, while in the connected scan stateduring roaming, the network station can evaluate communication qualityand load when the network station 5 is searching for another accesspoint (searching scan state) or when the network station is out-of-range(out-of-range scan state). Obviously, both the cost function weightfactor and the AP density setting threshold value ThV will be differentin the later cases.

[0092] In FIG. 4 a flow diagram of the roaming procedure for a networkstation (5, 6) in the present invention is shown.

[0093] In step 41 the Beacon frame of access point AP1 is received. Thevalues ATT of AP1, Average noise level NLAP of AP1 are stored by theprocessing unit 302 in memory device 304.

[0094] In step 42 the strength of the received signal is determined bythe processing unit 302 and stored in memory device 304. Also thenetwork station 5 determines its average noise level NLST for the accesspoint AP1.

[0095] In step 43, the processing unit in the MAC device 301 of thestation determines the value of ANL by comparing NLAP and NLST. If NLAPis larger than NLST, then the value of ANL becomes equal to NLAP, elseANL becomes equal to NLST. ANL is stored by processing unit 302 inmemory device 304.

[0096] In step 44, the processing unit in the MAC device 301 of thenetwork station chooses a threshold value ThV and a weight factor WEIGHTon basis of the scan state of the network station,

[0097] In step 45, the communication quality and load value CQL iscalculated using the cost function with the values of ATT, Signalreception level SRL, Average noise level ANL, ThV, and WEIGHT, which iscontinuously updated by the processing unit 302.

[0098] In step 46, the processing unit 302 compares CQL and ThV. If CQLis larger than ThV, the network station will stay connected to thepresent access point AP1. If so, the procedure continues in step 51.

[0099] In step 47, if CQL is smaller than ThV, the processing unit mustchange scan state and check whether the network station 5 isout-of-range.

[0100] In step 48, the processing unit starts a scanning procedure foranother cell while still in range. The procedure continues in step 50.

[0101] In step 49, the processing unit starts a scanning procedure whileout-of-range. The procedure continues in step 50.

[0102] In step 50, the network station 5 performs a search for availableaccess points based on the reception level of their signals and theirtraffic loads. The procedure will be described in more detail in FIG. 5.

[0103] In step 51 the processing unit exits the roaming procedure.

[0104] During normal association of the network station 5 with theaccess point AP1, the network station 5 also monitors all availableaccess points AP1, AP2, AP3 on a regular basis, to get information ontheir respective traffic load. Since traffic load per access point canvary strongly in time, due to the generally spiky characteristics ofdata transfers, the time interval on which the traffic load will beaveraged, must be chosen sufficiently large (e.g. 60s). In this loadbalancing procedure, similar to the roaming procedure, network station 5probes the Beacon frame of the access point AP1 and the Probe Responseframes of all available access points AP2, AP3. For each access pointAP1, AP2, AP3 the network station 5 gathers values for ATT, thresholdvalue ThV, WEIGHT, signal reception level SRL. For each access point(AP1, AP2, AP3) the average noise level ANL is determined as explainedabove from the values of NLAP and NLST. Additionally, the value of AUTT,the load of the network station 5 itself, is used as well. Using thecost function, the network station 5 will evaluate for each access pointAP1, AP2, AP3 the communication quality and load value CQL. For theaccess points AP2, AP3 to which the network station is not associated,the load of the network station 5 is added to the load of the accesspoint before calculating the cost function. The CQL value and thethreshold value ThV for each access point AP1, AP2, AP3 are stored in anarray format in memory device 304.

[0105] The processing unit 302 of the network station 5 now compares thestored communication and load values CQL for each access point AP1, AP2,AP3, and chooses the access point with the largest CQL value: thisaccess point has the best overall quality. If it is necessary at thistime to switch from one access point to another, the network station 5will disassociate from the former access point and associates with thelatter access point, according to the rules as defined by IEEE 802.11.

[0106] It is to be understood that all network stations 5, 6 in thenetwork, apply this load balancing procedure continually at fixed timeintervals (e.g. 60s) and as a result, the distributions of associationsacross the network will even out over time, with a better balance oftraffic load across the network and better overall data throughput. Byemploying a random starting time when a network station initialises, thenetwork station waits a random time before running the load balancingprocedure. This ensures that only a single network station 5, 6 runsthis procedure at a given time. Also, the CSMA/CA scheme will prohibitpossible collisions of transmissions in the network.

[0107] In FIG. 5 a flow diagram of the load balancing procedure for anetwork station in the present invention is shown.

[0108] In step 52, the network station performs a sweep to collectLBinfo from all available access points AP1, AP2, AP3, either throughthe Beacon frame or the Probe Response frame (after the station sendingthe Probe Request frame). For each access point AP1, AP2, AP3 the valuesATT, Average noise level NLAP, threshold value ThV, and the signalreception level SRL, are stored by the processing unit 302 in memorydevice 304. Also the network station 5 determines its average noiselevel NLST for EACH access point (AP1, AP2, AP3).

[0109] In step 53, the processing unit in the MAC device 301 of thestation determines the value of ANL by comparing NLAP and NLST. If NLAPis larger than NLST, then the value of ANL becomes equal to NLAP, elseANL becomes equal to NLST. For each access point (AP1, AP2, AP3) ANL isstored by processing unit 302 in memory device 304.

[0110] In step 54, the weight factor WEIGHT is set dependent on theconnection or scan state of the network station with respect to theaccess point to which it is associated.

[0111] In step 55, for each access point AP1, AP2, AP3 the communicationquality and load value CQL is calculated by the processing unit 302using the cost function with the values of ATT, Signal reception levelSRL, Average noise level ANL, ThV, WEIGHT. In case the network stationis not associated with the access point, the actual AUTT of the networkstation is added to the load value ATT of the access point. The valuesCQL are stored in an array format in memory device 304.

[0112] In step 56, the processing unit 302 compares the stored valuesCQL and calculates which access point has the largest CQL value.

[0113] In step 57, the processing unit checks if the access point withthe largest CQL value equals the access point the network station isalready associated with. If not, then the program continues in step 58,else if the best access point is the associated access point, theprogram continues in step 59.

[0114] In step 58, the network station changes its association from theold access point to the access point with better overall quality, usingthe disassociation and association procedures as defined by IEEE 802.11.The program continues in step 59.

[0115] In step 59, the timer for the load balancing procedure is resetto count down for the next execution of the procedure (e.g. after 60s).Here the load balancing procedure ends, the processing unit 302 returnsto the calling procedure.

[0116] Here an example is given of a possible scenario where loadbalancing can be useful. The access points AP1, AP2, and AP3 as shown inFIG. 1 give for the network station different signal reception levelsSRL and average noise levels ANL, and different traffic loads ATT. Inthis example a linear cost function will be used to keep matters simple,but it is to be understood that non-linear functions also may beapplied. Also for simplicity, the comparison of average noise levelsNLAP and NLST observed by an access point and the network station,respectively, is omitted here. The resulting values ANL, obtained asdescribed in step 43 of FIG. 4, are given here directly.

[0117] The cost function used here is given in equation 1.

CQL=(SRL−ANL)−WEIGHT*ATT  (1)

[0118] In the table below the network station's signal reception level(SRL) with respect to each access point, its average noise level (ANL)with respect to each access point (channel frequency used) and eachaccess point's averaged traffic load (ATT) are given. Access Point SRLANL ATT AP1 55 18 48 AP2 55 19 45 AP3 53 21  3

[0119] Clearly, the traffic load in the network is somewhat unbalanced.Assume the network station 5 is associated with access point AP1, in theconnected scan state. Further assume that the weight factor WEIGHT is0.5 (as an example, the value for the connected scan state), and theload of the network station AUTT is 10. The threshold value ThV is 30(30 dB signal-to-noise ratio). When the Beacon frame is received (step41), the processing unit will calculate the CQL value of the associatedaccess point using steps 42 and 44 according to the description of FIG.4. According to equation 1, the value of CQL is 13. In step 45,comparison of CQL with the threshold value ThV shows that CQL is smallerthan ThV, but the network station 5 is still in range of AP1. Now, instep 47 the processing unit 302 of the network station 5 will enter intoa searching scan state to look for other access points AP2, AP3 withpossibly a better overall quality. The procedure continues in step 52 ofFIG. 5. In step 54 WEIGHT is now set to a value of 0.25 (as an example,the value for the search scan state). In step 55 CQL for each accesspoint AP1, AP2, AP3 is calculated. For the associated access pointequation 1 is used. For the other access points AP2, AP3 the followingequation 2 is used in which the traffic load AUTT of the network station5 is added to the traffic load ATT of the access points.

CQL=(SRL−ANL)−WEIGHT* (ATT+AUTT)  (2)

[0120] In the following table the calculated CQL values for each accesspoint AP1, AP2, AP3 are given, with AUTT=10 and WEIGHT=0.25. AccessPoint CQL 1 25 2 22 3 28

[0121] In step 56, these different CQL values are compared. Based onthese numbers, the network station 5 will establish that the accesspoint AP3 provides the best overall quality. In step 57, it is checkedwhether the network station 5 is already associated with access pointAP3. If so, the timer is reset in step 59. If not, in step 58, thenetwork station 5 switches to access point AP3.

[0122] Finally, FIGS. 6a-6 f illustrate the effect of load balancing onthe roaming process of a mobile network station. In FIGS. 6a-6 f a mapis shown of an indoor environment with a width of 12 m and a length of50 m. In the environment two access points AP4 and AP5 are installed.Access point AP4 is installed at map-coordinates X=5 m, Y=5 m, andaccess point AP5 at X=25 m, Y=7 m. The scale of the building isindicated along the horizontal and vertical directions, wherein theX-direction denotes the horizontal direction, and the Y-direction thevertical direction. AP4 is using the channel at 2412 MHz, while AP5 isusing the channel at 2432 MHz. Both access points use a transmit powerof 35 mW. The average noise levels NLAP of the access points AP4 and AP5are assumed to be equal. Also the average noise level NLST of thenetwork station is assumed to be constant. The cell size of both accesspoints is configured for a setting of AP density high, with a cellsearch threshold of 30 dB. The traffic load on AP4 is 30 (percent),while the traffic load on AP5 is minimal (zero).

[0123] In FIG. 6a the signal reception level (in dBm) of access pointAP4 by a network station is shown as a contour map. FIG. 6b shows thecontour map of the signal reception level (in dBm) of AP5. In FIG. 6cthe difference in signal reception level (AP4 relative to AP5) is shown.In FIGS. 6a, 6 b, 6 c the level of each contour line is denoted by itsvalue. Without load balancing a mobile network station that is movingfrom a position close to AP4 towards AP5, will enter the search scanstate when the signal reception level difference becomes less than 8 dB.On the map this level is marked by dashed line L1 and line L2. Line L1marks the 8 dB difference in signal reception level going from alocation close to AP4 in the direction of AP5. Line L2 marks the 8 dBdifference in signal reception level when going in the oppositedirection.

[0124]FIGS. 6d, and 6 e show the combined communication quality and loadvalue CQL of access points AP4 and AP5, respectively as a contour map.FIG. 6f shows again the difference in signal reception level (in dB, AP4relative to AP5). In FIGS. 6d and 6 e the level of each contour line isdenoted by its CQL-value. In FIG. 6f the level of each contour line isdenoted by its value of the difference in signal reception level.

[0125] When using the load balancing procedure, the mobile station whichmoves from a location close to AP4 towards AP5, will start searching foran access point with better overall quality as soon as the CQL value isbelow 30. When the load on access point AP4 is minimal, the position ofthe mobile station for this condition is marked in FIG. 6d by thecontour line 30 with level 30. However, due to the load on AP4 (loadvalue: 30), the transition to the search scan state will shift withrespect to this contour line. When the simple linear cost function ofequation 1 is used with a weight factor WEIGHT=0.5, the CQL value of 30is already reached at contour line 45 (level 45) in FIG. 6d. The searchscan state zone is now shifted towards the busier access point AP4 andis reached already at a higher signal reception level of AP4. As shownin FIG. 6f the lines marking the searching scan state criterion aregiven as L3 and L4. When looking at the difference in signal receptionlevel, the search scan state is entered at approximately 15 dB (L3,going from AP4 towards AP5), and at approximately 7 dB (L4, going in theopposite direction).

[0126] By using this procedure, the effective cell size around accesspoints with a high traffic load will shrink. Mobile network stationswill start roaming earlier to an access point with less traffic load.The present invention yields a better overall throughput in a network bythe combined load balancing and roaming scheme.

What is claimed:
 1. A communication system comprising a plurality ofaccess points (AP1, AP2, AP3) and at least one network station (5, 6),said at least one network station (5, 6) being arranged to communicatewith one of said plurality of access points (AP1, AP2, AP3) through awireless communication protocol, wherein each of said plurality ofaccess points (AP1, AP2, AP3) is arranged to: monitor its access pointtraffic load and transmit an access point traffic load parameter (ATT)indicative of said access point traffic load to said at least onenetwork station (5, 6), and said at least one network station (5, 6) isarranged to: monitor its network station traffic load; store a networkstation traffic load parameter (AUTT) indicative of said network stationtraffic load; receive access point traffic load parameters (ATT) fromsaid plurality of access points (AP1, AP2, AP3); select a communicationconnection with one of said plurality of access points (AP1, AP2, AP3)using a predetermined cost function taking at least said access pointtraffic load parameters (ATT) and said network station traffic loadparameters (AUTT) into account.
 2. A system according to claim 1,wherein each of said plurality of access points is arranged to determineits access point traffic load parameter by monitoring its transmissionand reception time load averaged over time.
 3. A system according toclaim 2, wherein each of said plurality of access points is arranged totransmit its access point traffic load parameter in an additional dataframe within a beacon and probe response frame.
 4. A system according toany of the claim 3, wherein each of the plurality of access points (AP1,AP2, AP3) is also arranged to: monitor its communication quality withsaid at least one network station (5, 6), and to transmit an accesspoint communication quality parameter (NLAP) indicative of saidcommunication quality to said at least one network station (5, 6); andsaid at least one network station (5, 6) is arranged to: receive saidaccess point communication quality parameter (NLAP) from a predeterminedaccess point, determine during reception of signals a network stationcommunication quality parameter (NLST) indicative of the communicationquality for said network station (5, 6) with said predetermined accesspoint, select the largest of said communication quality parameters(NLAP, NLST) as a minimum communication quality parameter (ANL), selecta communication connection with one of said plurality of access points(AP1, AP2, AP3) using said predetermined cost function taking also saidminimum communication quality parameter (ANL) into account.
 5. A systemaccording to claim 4, wherein said access point communication qualityparameter is calculated as average noise level (NLAP), said networkstation communication quality parameter is calculated as average noiselevel (NLST), and said minimum communication quality parameter iscalculated as average noise level (ANL).
 6. A system according to anyclaim 1, wherein said at least one network station (5, 6) is arranged todetermine its network station traffic load parameter by monitoring itstransmission and reception time load averaged over time (AUTT).
 7. Asystem according to claim 1, wherein said at least one network station(5, 6) monitors its signal reception level (SRL) and is arranged toselect a communication connection with one of said plurality of accesspoints (AP1, AP2, AP3) using said predetermined cost function takingalso said signal reception level (SRL) into account.
 8. A systemaccording to claim 1, wherein said cost function uses a weight factor tomodify relative weight of said access point traffic load parameter(ATT).
 9. A system according to claim 8, wherein said weight factor isalso used to modify relative weight of said network station traffic loadparameter (AUTT).
 10. A system according to claim 8, wherein said weightfactor has a value in dependence on scan state of said at least onenetwork station (5, 6).
 11. A system according to claim 10, wherein saidscan state is either one of the following states: connected scan state,searching scan state, or out-of-range scan state.
 12. An access point(AP1, AP2, AP3) arranged to communicate with at least one networkstation (5, 6) through a wireless communication protocol, wherein saidaccess point (AP1, AP2, AP3) is also arranged to: monitor its accesspoint traffic load and transmit an access point traffic load parameter(ATT) indicative of said access point traffic load to said at least onenetwork station (5, 6).
 13. An access point according to claim 12,arranged to determine its access point traffic load parameter bymonitoring its transmission and reception time load averaged over time.14. An access point according to claim 12, wherein said access point isarranged to transmit its access point traffic load parameter in anadditional data frame within a beacon and probe response frame.
 15. Anaccess point according to claim 12, wherein the access point (AP1, AP2,AP3) is also arranged to: monitor its communication quality with said atleast one network station (5, 6), and to transmit an access pointcommunication quality parameter (NLAP) indicative of said communicationquality to said at least one network station (5, 6).
 16. An access pointaccording to claim 15, wherein said access point communication qualityparameter is average noise level (NLAP).
 17. A network station (5, 6)arranged to communicate with one of a plurality of access points (AP1,AP2, AP3) through a wireless communication protocol, wherein saidnetwork station (5, 6) is arranged to monitor its network stationtraffic load; store a network station traffic load parameter (AUTT)indicative of said network station traffic load; receive access pointtraffic load parameters (ATT) indicative of access point traffic loadsfrom said plurality of access points (AP1, AP2, AP3); select acommunication connection with one of said plurality of access points(AP1, AP2, AP3) using a predetermined cost function taking at least saidaccess point traffic load parameters (ATT) and said network stationtraffic load parameters (AUTT) into account.
 18. A network station (5,6) according to claim 17, arranged to receive from said predeterminedaccess point from said plurality of access points (AP1, AP2, AP3) anaccess point communication quality parameter (NLAP) indicative of thecommunication quality of said predetermined access point, to determinefor said predetermined access point a network station communicationquality parameter (NLST) indicative of the communication quality forsaid network station (5, 6) with said predetermined access point, toselect for said predetermined access point the largest of saidcommunication quality parameters (NLAP, NLST) as a minimum communicationquality parameter (ANL), and to select a communication connection withone of said plurality of access points (AP1, AP2, AP3) using saidpredetermined cost function taking also said minimum communicationquality parameters (ANL) into account.
 19. A network station accordingto claim 18, wherein said access point communication quality parameteris calculated as average noise level (NLAP), said network stationcommunication quality parameter is calculated as average noise level(NLST), and said minimum communication quality parameter is calculatedas average noise level (ANL).
 20. A network station according to claim17, arranged to determine its network station traffic load parameter bymonitoring its transmission and reception time load averaged over time(AUTT).
 21. A network station according to claim 17, arranged to monitorits signal reception level (SRL) and to select a communicationconnection with one of said plurality of access points (AP1, AP2, AP3)using said predetermined cost function taking also said signal receptionlevel (SRL) into account.
 22. A network station according to claim 17,wherein said cost function uses a weight factor to modify relativeweight of said access point traffic load parameter (ATT).
 23. A networkstation according to claim 22, wherein said weight factor is also usedto modify relative weight of said network station traffic load parameter(AUTT).
 24. A network station according to claim 22, wherein said weightfactor has a value in dependence on scan state of said at least onenetwork station (5, 6).
 25. A network station according to claim 24,wherein said scan state is either one of the following states: connectedscan state, searching scan state, or out-of-range scan state.
 26. Amethod in a communication system comprising a plurality of access points(AP1, AP2, AP3) and at least one network station (5, 6), forcommunication between said at least one network station (5, 6) and oneof said plurality of access points (AP1, AP2, AP3), said at least onenetwork station (5, 6) being arranged to communicate with one of saidplurality of access points (AP1, AP2, AP3) through a wirelesscommunication protocol, wherein said method comprises the followingsteps carried out by the access points (AP1, AP2, AP3): monitoringaccess point traffic loads and transmitting access point traffic loadparameters (ATT) indicative of said access point traffic loads to saidat least one network station (5, 6), and said method also includes thefollowing steps carried out by said at least one network station (5, 6):monitoring network station traffic load; storing a network stationtraffic load parameter (AUTT) indicative of said network station trafficload; receiving access point traffic load parameters (ATT) from saidplurality of access points (AP1, AP2, AP3); selecting a communicationconnection with one of said plurality of access points (AP1, AP2, AP3)using a predetermined cost function taking at least said access pointtraffic load parameters (ATT) and said network station traffic loadparameters (AUTT) into account.
 27. A method carried out by an accesspoint (AP1, AP2, AP3) arranged to communicate with at least one networkstation (5, 6) through a wireless communication protocol, wherein saidmethod comprises the following steps: monitoring access point trafficload and transmitting an access point traffic load parameter (ATT)indicative of said access point traffic load to said at least onenetwork station (5, 6).
 28. A computer program product to be loaded byan access point (AP1, AP2, AP3) arranged to communicate with at leastone network station (5, 6) through a wireless communication protocol,wherein said computer program product allows said access point to carryout the following steps: monitoring access point traffic load andtransmitting an access point traffic load parameter (ATT) indicative ofsaid access point traffic load to said at least one network station (5,6).
 29. A data carrier provided with a computer program productaccording to claim
 28. 30. A method to be carried out by a networkstation (5, 6) arranged to communicate with one of a plurality of accesspoints (AP1, AP2, AP3) through a wireless communication protocol,wherein said method comprises the following steps: monitoring networkstation traffic load; storing a network station traffic load parameter(AUTT) indicative of said network station traffic load; receiving accesspoint traffic load parameters (ATT) indicative of access point trafficloads from said plurality of access points (AP1, AP2, AP3); selecting acommunication connection with one of said plurality of access points(AP1, AP2, AP3) using a predetermined cost function taking at least saidaccess point traffic load parameters (ATT) and said network stationtraffic load parameters (AUTT) into account.
 31. A computer programproduct to be loaded by a network station (5, 6) arranged to communicatewith one of a plurality of access points (AP1, AP2, AP3) through awireless communication protocol, wherein said computer program productallows said network station to carry out the following steps: monitoringnetwork station traffic load; storing a network station traffic loadparameter (AUTT) indicative of said network station traffic load;receiving access point traffic load parameters (ATT) indicative ofaccess point traffic loads from said plurality of access points (AP1,AP2, AP3); selecting a communication connection with one of saidplurality of access points (AP1, AP2, AP3) using a predetermined costfunction taking at least said access point traffic load parameters (ATT)and said network station traffic load parameters (AUTT) into account.32. A data carrier provided with a computer program product according toclaim 31.